BUG: Ignored write errors in PGP signature generation (writeMPI)

[andrinoff]

Describe the bug

In pgp/yubikey.go lines 386-387, the writeMPI() function ignores write errors using //nolint:errcheck:

w.Write(buf)  //nolint:errcheck
w.Write(data) //nolint:errcheck

These write operations could fail silently if buffer capacity is reached or I/O errors occur, producing invalid OpenPGP signatures without error indication.

To reproduce

1. Trigger PGP signature generation with Yubikey
2. If buffer writes fail (simulated via full buffer), signatures corrupt silently
3. No error returned to caller

Expected behavior

Write errors should be checked and propagated:

if _, err := w.Write(buf); err != nil {
    return err
}
if _, err := w.Write(data); err != nil {
    return err
}

Screenshots

N/A

Additional context

• File: pgp/yubikey.go
• Lines: 386-387
• Severity: High - silent signature corruption possible
• Fix complexity: Easy - add error checks and propagate